Controlled burning of potent solid oxidizer



United States ate 3,373,%3 Patented Mar. 12, I968 Nitronium perchlorate, NO CIO is a potent solid oxidizer that has been very difiicult to utilize in propellant formulations. The present invention is concerned with accomplishing a controlled burning of nitronium perchlorate by admixing with the nitronium perchlorate small critical amounts of compatible fuel substances (polymers, metals, waxes and the like) that cause the burning to be effected at a controlled rate.

All attempts to burn nitronium perchlorate (NPC) at ordinary rocket chamber pressures, e.g. 500 psi and higher in balanced systems with high ener y fuels have resulted in rapid deflagration or detonation. Pure NPC will not burn at ordinary rocket chamber pressures and a large amount of heat must be supplied to cause it to decompose.

The fact that NPC reacts violently with high-energy fuels, e.g. fuels rich in hydrogen, such as hydrazine bisborane, makes necessary the encapsulation of the NPC to prevent its contact with high-energy fuel. Encapsulation of small granules of the NPC is expensive, often ineffective, and incurs hazards and problems of control when the coated granules are mixed with a high energy fuel. On the other hand, if large compressed masses or shaped forms of NPC are to be used, while they are readily macroencapsulated with a compatible coating, e.g. polymer film or aluminum foil, it is desirable to have these large masses of encapsulated NPC burn at the same rate as the other propellant matrix or fuel in order to insure adequate and controlled combustion during the burning period available in the combustion chamber.

Compression of nitronium perchlorate crystals into a high density grain for macroencapsuiation is set forth in U.S. application Ser. No. 66,672 filed Nov. 1, 1960, by H. Bieber and J. Lakritz.

It has now been found that, although NPC by itself is difiicult to burn, by adding a small but critical quantity of a compatible fuel finely divided and well mixed with the NPC, burning of the mixture takes place at a controlled rate. The rate of burning of the mixture depends on the particle size of the NPC, the particle size of the admixed compatible fuel and the amount of added finely divided compatible fuel. The pressure exponent of the NPC-fuel mixture must also be consistent with that of the other (fuel-rich) propellant grain. The pressure exponent .n is the exponential variation of burning rate, r, with pressure, pzr=ap where a is a constant.

The compatible fuels suitable for admixture in critical amount with the NPC crystals are substances that can be admixed under ordinary conditions without causing appreciable reaction of deflagration or detonation, and which under the combustion temperature and pressure conditions obtaining after ignition in a rocket combustion promote the desired burning of the NPC. Examples of compatible fuels useful for this purpose are metal powders (e.g., Al, B, Be Mg), substantially saturated hydrocarbon polymers such as polyethylene (-CH and other solid or semisolid hydrocarbonaceous compounds, herein abbreviated (C H which are mostly saturated hydrocarbon, e.g., waxes, saturated fatty acids and saturated fatty esters containing a long saturated hydrocarbon chain of polymethylene groups (CH Any of the finely divided compatible fuel substances which may be used, has a particular effect on the rate of burning and the energy output, but in accordance with the principles set forth herein, one skilled in the art can select the kind of material best suited.

The following example illustrates factors involved in using polyethylene powder of micron size for admixing with NPC to cause the mixture to burn at a controlled rate.

EXAMPLE TABLE I.-CONTROLLED NO ClO BURNING Wt. Percent; Polyethylene Burning Rate, Inches/Sec.

5-15 Microns NPC 100-200 Microns NPC It can be seen from the data in Table I that by homogeneously admixing the polyethylene in a critical amount of 2 to 5 wt. percent with the NPC, a satisfactory burning rate is obtained for the mixtures containing about 2 to 5 wt. percent of the polyethylene powder. Without the aid of polyethylene powder there is no burning of the NPC under the test conditions using the same temperature for ignition. The preferred concentration of the admixed powdered fuel is lower when the powdered fuel is smaller in size, e.g., 5-15 microns particle diameter. It is of interest to note that the region of rapid deflagration (starting at the 6% level for the larger particle size NPC) is far short of stoichiometric proportions for a quantity of the fuel which would give balanced CO and H gas formation. 35 wt. percent hydrocarbon is the quantity of fuel that must be added to the NPC to give a stoichiometric balance. As is shown in the table, the burning is out of control even at a 4 wt. percent (-CH level for the smaller NPC particles and shows signs of reaching inordinately high burning rates above the 5% (CH level with the l00200 microns NPC.

In plotting data on the burning rate vs. weight percent of fuel admixed in critical amounts between about 2 and 5 wt. percent with the NPC, smooth curves were obtained showing that the burning rate is also a function of the pressure and the fuel type. The data taking into consideration these factors as well as the factors shown in the table are correlated by the expression: r=acp where a is a parameter depending on the type of fuel, ambient temperature, the particle size of the NPC and of the fuel, c is the fuel concentration in the NPC-fuel phase, and n is a pressure exponent depending on fuel type and NPC particle size.

While polyethylene is an example of a substantially saturated hydrocarbon that acts as a satisfactory compatible fuel with respect to the NPC, suitable control in burning of NPC with an admixed petroleum resin in the range of 2 to 4 wt. percent was also obtained. This petroleum resin may have as many as four double bonds per 100 carbon atoms. The petroleum resin has a complex structure since it is made by catalytic polymerization of a mixture of iso-olefins diolefins in the C to C. boiling range. The mixed hydrocarbons subjected to polymerization may contain benzene and toluene. The predominant reactants that form the petroleum resin are iso, mono-olefins and diolefins which are copolymerized and the long carbon chain may contain some side chain hydrocarbon groups.

The kind of rocket system in which the potent NPC oxidizer makes a valuable propellant is one in which a fuel-rich monopropellant, such as hydrazine bisborane (BH N H.,BH is used provided the NPC is protected from moisture and from the HBB and other reactive metals. In accordance with the present invention, the NPC powder is homogeneously mixed with the critically small amount of compatible fuel needed for promoting controlled burning. This homogeneous mixture is compressed and macroencapsulated by a metal foil (Al, Mg, or B), by a compatible hydrocarbon polymer, or both to keep the NPC mixture protected and separated from the noncompatible hydrogen-rich fuel, such as Then by proportioning the oxidizer and fuel ingredients the solid oxidizer mixture can be made to burn at a satisfactory rate equivalent to the rate at which the separated high energy fuel burns as a monopr-opellant. The product gases from both the fuel and the NPC mixture mix and react further before being ejected through the rocket nozzle. The final gas composition is therefore the same as would have been obtained from a well-mixed HBB'NPC propellant (if such a propellant were possible). Most of the impulse is derived from this latter step of comixing and burning the oxidizer-rich (from NPC) and fuel-rich (from HBB) gases. Various arrangements can be made of the encapsulated NPC mixture with respect to the fuel in the combustion chamber for certain advantages as set forth in US. application Ser. No. 79,916, filed Dec. 30, 1960 by J. P. Longwell et al., and US. application Ser. No. 37,514, filed June 20, 1960 by I. P. Longwell.

As indicated, a mixture of NPC and HBB in intimate contact undergoes explosive reaction. The following systems show the relative amounts of an NPC mixture having a controlled burning rate and of HBB where an Al foil partitioning wall separates the NPC from the high energy fuel. This is System I.

Note that here, according to the teachings of this invention, one could not mix all the (Cl-I polymer with the NPC, since this would give 8% fuel (on NPC) and result in explosive burning. Conversely, if all the (CH were used as binder for the HBB, the NPC grain would not burn but merely ablate away at a rate far below the burning rate of HBB (0.5 in./sec. at 500 p.s.i. Now, it is seen from Table I that about 3.3% (CH (on NFC) gives a burning rate of 0.5 in./ sec. Thus, 2.2 parts of the (CH are used as binder for the 63.5 parts of NPC (2.2/65.7=3.3%), and 2.8 parts of the (CH are used as binder for the HBB. This amount of (CH will not affect the burning rate of BBB since the latter is a monopropellant in its own A. right (N H B H 2BN+5H while NPC by itself cannot undergo exothermic decomposition. Also, if the formulation calls for more metal than can be utilized in the metal foil grain separation layers, then said excess metal is incorporated as powder in the fuel (or HBB) phase.

Here in System II, by a similar reasoning, 2.5 parts of the (CH polymer or hydrocarbon will \be incorporated as binder in the NPC phase and 2.9 parts of (CH polymer in the HBB phase in order to give equivalent burning rates.

In the foregoing representative systems, the small amount of polymer used for controlling the NPC burning rate and even the small amount of admixed aluminum powder or aluminum liner does not detract appreciably from the overall performance value for the systems with regard to lsp or energy released incomparison to a system which omits both the powdered metal and the relatively low energy polymer. In fact the metal may even enhance performance over the base case.

The NPC given the controlled burning rate by the added hydrocarbon-containing material is made more useful in high impulse hybrid rockets Where a separate fuel-rich (i.e. hydrogen rich) monopropellant is employed as a separate phase. The controlled burning-rate potent oxidizer may also be used in various high-energy gas generating equipment. This startling, yet reproducible, variation in burning rate of NPC grains with admixture of such limited concentrations (far removed from usual stoichiometric propellant formulations) of binder is an invention entirely unexpected in view of the present art in rocket fuel fabrication.

What is claimed is:

1. Nitronium perchlorate mixed with a critical quantity in the range of about 2 to 5 wt. percent of finely divided, solid, hydrocarbonaceous material which is principally polymethylene in composition.

2. A solid oxidizer of composition defined in claim 1 in which the hydrocarbonaceous material is polyethylene.

3. A solid oxidizer grain consisting essentially of nitronium perchlorate powder mixed with about 2 to 5 wt. percent of polyethylene powder to give the resulting mixture a burning rate of about 0.5 inch per second under pressure of 500 p.s.i.

References Cited UNITED STATES PATENTS 10/1961 Fox ct al. 149-19 2/1961 Fox 52-05 OTHER REFERENCES BENJAMIN R. PADGETI, Primary Examiner.

LEON D. ROSDOL, ROGER L, CAMPBELL,

Examiners, 

1. NITRONIUM PERCHLORATE MIXED WITH A CRITICAL QUANTITY IN THE RANGE OF ABOUT 2 TO 5 WT. PERCENT OF FINELY DIVIDED, SOLID, HYDROCARBONACEOUS MATERIAL WHICH IS PRINCIPALLY POLYMETHYLENE IN COMPOSITION.
 2. A SOLID OXIDIZER OF COMPOSITION DEFINED IN CLAIM 1 IN WHICH THE HYDROCARBONACEOUS MATERIAL IS POLYETHYLENE. 